SQL Goodies

May 18, 2016

I’ve recently been diving into SQL (postgres to be exact). Here are a few random tidbits that have helped me on my journey.

Things to read

Don’t read blogs (except mine 😉) While there are many helpful resources on the internet for learning about SQL, I’ve found the scattershot approach of reading blogs to be less than helpful in my own journey. They often cover only part of a subject or are too focused on a specific dialect to be truly helpful. I’m not suggesting that articles are never helpful, but that they can remove clarity when you are starting out. Longer form material is much better suited at the early stages.

To that end, I’ve found Learning SQL to be the best comprehensive guide on SQL; it includes a sample database and exercises with each chapter that provide a great way to exercise the concepts you read. There are plenty of long-form books out there, but this was the most approachable and comprehensive for basic concepts.

Use a dataset that matters to you. If you workplace provides a sanitized read-only copy of data, use it! If that doesn’t spark your interest, create your own database (favorite sports tems, ninja clans, etc), or convert something from data.gov or other data providers). I find it much easier to work with data that I have an emotional or business connection to.

An ORM should be a jetpack, not a crutch

ORMs like ActiveRecord are fantastic, they enable teams to write easy, reusable queries without littering your code with verbose SQL. Unfortunately, this means that when you need to dip down into raw SQL or troubleshoot a SQL problem, you can be left out in the cold.

Most ORMs provide a query logging feature, or a way to translate a series of method calls into an SQL statement. In active record, this is the to_sql method, e.g. MyModel.joins(:my_association).where(query: true).to_sql. Using this can help you understand and translate complicated ORM logic into SQL to better understand what is going on under the hood.

Ensure that you understand the structure of your database by looking at it in a console (or a schema file, if your ORM provides one). Even if you are using an ORM, this will make the connections between objects much easier to visualize. It also helps demystify abstract and potentially confusing ORM concepts like PolyMorphic Associaitons when you see what they are doing under the hood.

Once you have a feel for the structure of a database, use toy projects or a throwaway branch to selectively replace ORM code with raw SQL. Many ORMs will give you an ability to mix raw SQL in with their own wrappers. E.g. ActiveRecord will allow you to select off of queries, or execute a raw sql statement with ActiveRecord.base.connection.execute. This provides a great way to gradually start using SQL without jumping in head first (if that’s not your cup of tea). It’s also a great way to see how an ORM may optimize (or not optimize) your queries.

The more you learn about SQL (and how your ORM formats SQL) the better your code (whether raw or ormified) will be.

Console tips

The SQL console is one of the most useful tools for learning SQL and discovering your dataset. Here are a few tricks and tips

explain to see what your queries are doing under the hood

show tables (or \d for Postgres) gives you your DB’s structure

describe <table> (or \d <table> for PG) gives you a table’s structure

\e in a console will open the current command in the editor of your choice. Very handy for tweaking long queries

Postgres Only

\x Use super pretty “extended display” for outputting queries

\i file.sql (PG only) executes a query from a file

\o output.out writes output to output.out which can be great for storing the results of a query for later